The Invention relates generally to parabolic reflector-type solar collectors which concentrate solar energy along a focal line, and in particular to a parabolic reflector body construction suitable for in situ, large-scale fabrication, together with a method for making the same.
Elongated, parabolic solar collectors employ a concave trough-like reflective surface having a parabolic shape to focus the sun's energy onto a focal line. Such collectors harness solar energy by positioning an absorber pipe or similar energy transfer device along the focal line of the parabolic reflector. Concentrated solar energy heats the water or other transfer medium in the absorber pipe, which is then transferred to a generator or similar device to perform useful work.
Linear parabolic collectors are rotationally mounted and pivoted mechanically to follow the diurnal movements of the sun. Installations of linear parabolic collectors date back to the early part of this century. One example is the solar-powered pumping station built by Mr. Frank Shuman in Meadi, Egypt, in 1913. Today, large-scale linear parabolic collectors could provide an economically viable alternative to conventional power generation systems, if large-scale reflector structures with sufficient structural integrity and stability can be constructed at a cost low enough to justify the required financial investment. Roughly two-thirds of the cost of harnessing solar energy by means of parabolic solar collectors is the construction of the large numbers of collectors required. The cost of each collector structure must be reduced if a favorable cost-to-benefit ratio is to be achieved for solar energy. The twin goals of reducing cost and increasing benefits for a solar collector installation will need to be met before solar energy becomes an economically important energy resource.
Designing and building solar power stations for generating electricity is a complex problem requiring large-scale, efficient collectors at locations which receive maximum sunlight. Two alternatives for providing collectors at high-sunlight locations are (1) to construct collectors at the site, typically a remote location without fabrication facilities, or (2) to construct the collectors elsewhere and transport them to the site. The required size of linear parabolic solar collectors make their transport impractical.
Fabricating large-scale linear parabolic collectors out of steel framing is not only time consuming and expensive but also subject to introduction of optical distortion due to fabrication stresses. Maximum collector efficiency requires exceptionally accurate parabolic reflective surface, which in turn requires high-tolerance construction techniques. Structures supporting a parabolic reflector-type collector must be both well designed and accurately assembled to produce and maintain its parabolic shape under thermal conditions which vary widely throughout its useful life. Weather conditions, temperature extremes and corrosion represent a constant challenge to the integrity of any exposed, steel-frame structure. Assembly of a frame-type collector at the site of a power station requires transport of a large volume of material to the site and accurate assembly under sometimes harsh conditions.
It would be advantageous to be able to fabricate highly accurate, free-standing parabolic collectors in remote locations without the need to transport all structural materials to the site. It would also be advantageous to provide linear parabolic collectors which possess a high degree of internal strength and structural integrity to resist changes in shape under various thermal conditions and adverse weather conditions. It would be particularly advantageous to provide parabolic solar collectors which can be fabricated economically enough to compete, on a per Kilowatt-Hour (kWH) basis, with conventional thermal energy sources such as fossil fuels and nuclear power.
It is an object of the present invention to provide a parabolic reflector body of the type which concentrates solar energy along a focal line, for use in linear parabolic collectors, and which can be fabricated on an elongated convex parabolic mold near where the solar collector is to be permanently installed.
It is another object of the invention to provide a method of fabricating parabolic solar collectors of the type which concentrate solar energy along a focal line. The method includes forming an elongated mold having a convex top surface with a parabolic cross-section and then forming an elongated layered structure and associated support structures on the mold.
Accordingly, the invention provides a parabolic reflector body fabricated on an elongated, convex parabolic mold, the reflector body being of the type which concentrates solar energy along a focal line. The parabolic reflector body comprises an elongated layered structure having a parabolic concave first side, a convex second side opposite the first side, and generally parallel longitudinal edges. The layers of the structure include a reflective layer on the concave first side which contacts and is conformed to the mold during fabrication, and a support layer bonded to the first layer on the convex second side. The support layer is predominately formed of an amorphous hard-curing adherent material applied over the reflective layer in a moldable state and conformed generally to the convex shape of the mold. In addition, the reflector body includes a beam member bonded to the layered structure on the second side thereof, extending longitudinally parallel to the side edges of the layered structure. In its preferred form, the reflector body includes a tubular beam member which is embedded into a ridge of amorphous hard-curing adherent material applied on the support layer. Additional structural members are provided, extending generally between the beam member and the layered structure, to further support the reflector body.
The invention further provides a means for fabricating a parabolic reflector body, the means being an intermediate construction formed during the process of fabricating the completed reflector body. The means for fabricating comprises the mold on which the layered structure of the reflector body is fabricated, together with the layered structure itself. The mold of the present invention is an elongated mold having a convex top surface with a parabolic cross-section. The elongated reflector body fabricated on the mold, which body is detachable therefrom after fabrication, includes a layered structure having a concave parabolic first side, a convex second side and generally parallel longitudinal edges. The layers of the body include a reflective layer on the concave first side in contact with and conforming to the mold, and support layer, bonded to the reflective layer. The support layer is predominately formed of an amorphous, hard-curing adherent material applied over the reflective layer in a moldable state and shaped to generally conform to the mold. In its preferred form, the means for fabricating a parabolic reflector body includes a beam member extending longitudinally along the support side of the layered structure, generally parallel to and centrally disposed between the longitudinal edges. A central ridge of amorphous, hard-curing adherent material in a moldable state is preferably centrally disposed on the support layer, together with similar parallel edge ridges extending longitudinally adjacent the edges of the layered structure. The beam member is partially embedded in the central ridge. Additional structural members are partially embedded in the edge ridges and are preferably also embedded in mortar extending along the beam member.
The invention further provides a method of fabricating a parabolic reflector body of the type which concentrates solar energy along a focal line. The method comprises steps which include forming an elongated mold having a convex top surface with a parabolic cross-section. Steps in forming the elongated mold include providing a hard-curable moldable material on a base surface, such as the ground. A screed is then guided over the moldable material by means of a screed-moving apparatus. The screed has a parabolic shape and is moved longitudinally along the base surface supporting the mold. After the screed has shaped the mold, the moldable material is allowed to cure.
Following formation of the mold, an elongated layered structure is formed on the mold by steps including: applying a reflective layer of material having a reflective surface on the top of the mold with the reflective surface facing down; positioning a lattice of reinforcing members over the reflective layer; and forming a support layer on the reflective layer. The support layer is formed by steps including applying an amorphous, hard-curing adherent material in a moldable state onto the reflective layer to a depth which covers the reinforcing members. The support layer is then shaped by guiding a screed over the amorphous, hard-curing adherent material by means of the screed-moving apparatus used to form the mold, whereby the reflective and support layers together form the layered structure of the reflector body.
In its preferred form, the method further includes bonding a beam member longitudinally to the support layer of the layered structure, after the shaping of the support layer. Additional preferred steps include forming a central ridge of amorphous, hard-curing adherent material on the support layer centrally on the second side of the layered structure and forming two additional parallel ridges adjacent the edges of the layered structure. The beam member is then embedded into the central ridge of amorphous hard-curing adherent material and additional structural members are partially embedded in the edge ridges and similarly embedded in mortar extending along the beam member. After curing, the reflector body is an integral, self-supporting unit with a highly accurate parabolic, concave front surface.